1. Field of the Invention
The present invention relates to a radio-frequency magnetic field regulating apparatus which is used as supplementary means in a magnetic resonance diagnostic apparatus such as a magnetic resonance imaging apparatus for obtaining morphological information of a subject to be examined (living organism) and a magnetic resonance spectroscopic imaging apparatus for obtaining spectroscopic functional information utilizing magnetic resonance phenomenon.
2. Description of the Related Art
The magnetic resonance phenomenon is a phenomenon in which atomic nuclei placed in a static magnetic field and having non-zero spins and magnetic moments absorb and emit electromagnetic-wave energy at specific resonant frequencies. The atomic nuclei are excited to resonance at the Larmor angular frequency given by EQU .omega.o=.gamma.Ho
where .gamma. is the gyromagnetic ratio inherent in the atomic nuclei and Ho is the strength of the static magnetic field.
The apparatus adapted to make in vivo diagnosis utilizing the above principle performs signal processing on electromagnetic waves having the same frequency as above and induced after the resonance absorption, thereby obtaining diagnostic information reflecting atomic nucleus density, longitudinal relaxation time T1, transverse relaxation time T2, flow, and chemical shifts on a noninvasive basis, for example, cross-sectional magnetic resonance images of an object to be examined.
To acquire diagnostic information utilizing magnetic resonance, the whole body of a human subject placed in a static magnetic field may be excited to acquire magnetic resonance signals from the whole body. In view of constraints on construction of apparatus and clinical requirements for magnetic resonance images, however, actual apparatuses are adapted to excite a specific body region of the subject and acquire magnetic resonance signals from the body region.
In this case, the specific imaging region is usually chosen to be a slice of a certain thickness. Magnetic resonance signals (MR signals) such as echo signals and FID signals are acquired by repeating a data encoding process many times. These data groups are subjected to, for example, two-dimensional Fourier transform for image reconstruction, thereby producing an cross-sectional MR image of the selected slice of the subject.
In the magnetic resonance imaging diagnostic technology, a specific body region is generally defined by a radio-frequency magnetic field (an RF pulse) produced by a radio-frequency magnetic field producing coil and gradient magnetic fields produced by gradient magnetic field producing coils.
In such a magnetic resonance diagnostic apparatus, when a diagnosis is made of, for example, the abdomen of a human subject, a radio-frequency magnetic field is produced by a whole-body radio-frequency magnetic field producing coil which is incorporated into the apparatus in order to exert influence of the radio-frequency magnetic field on a relatively wide range.
The subject to be examined is placed within a RF coil system such as a saddle-shaped whole-body coil. The position and thickness of a selected slice for diagnosis are determined by a radio-frequency magnetic field (an RF pulse) produced by the whole-body coil and gradient magnetic fields Gx, Gy and Gz produced by gradient magnetic field producing coils.
In this case, the spatial distribution of the radio-frequency magnetic field is determined by coil characteristics such as the coil pattern shape, the distribution and capacitances of distributed capacitors, etc., and the conductivity, dielectric constant, etc., of the subject to be examined. For this reason, when such a large coil (whole-body coil) is used, ununiformity will be produced in the radio-frequency magnetic field distribution because of the influence of the coil characteristics, etc. In addition, the spatial distribution of radio-frequency magnetic field will be distorted (become ununiform) within the subject because of its conductivity, permittivity, permeafility, and boundary condition depending on the radio-frequency magnetic field characteristics, produced by the coil. The phenomenon which is described become the difference of the spatial transmitting sensitivity at the transmitting and receiving sensitivity at the receiving. These factors could produce irregularities in sensitivity in obtained MR images. In a magnetic resonance imaging apparatus utilizing such a high magnetic field as 1.5T, in particular, the phase of magnetized spins is liable to vary because of movement phenomena such as the motion of the heart. In a high-magnetic-field magnetic resonance imaging apparatus, because the apparatus have the characterization is sensitive to vary since the spatial magnetic susceptibility, what is referred to as susceptibility artifacts become marked. The frequency value of the radio-frequency magnetic field for using reception and/or transmission, become large number (nealy 60 MHz), become to neglest the distortion of the radio-frequency magnetic field at the receiving based on the effect such as the shape of the subject, the conductivity and the dielectric constant. For example, when an abdomen is photographed by heart cine-photographing, the left ventricle heart muscle portion is blackened as indicated by oblique lines in FIG. 1 which is an axial sectional view of the abdomen, failing to make highly exact diagnosis.
In order to solve such a problem, it is required to partly control the radio-frequency magnetic field distribution in the space within the coil. For example, adjusting coil characteristics or transmit signals to be applied to the coil for each imaging condition may be conceived. However, this will require hardware or software to be adjusted for each imaging condition. Adjusting hardware or software for each imaging condition is too troublesome to be done practically.